Could a spacecraft refuel from the atmosphere?

If it dipped into the atmosphere temporarily on an elliptical orbit, could the air gathered be stored for a long flight? Then maybe get more fuel from another planet's atmosphere or moon for the return trip?

Staff: Mentor

If it dipped into the atmosphere temporarily on an elliptical orbit, could the air gathered be stored for a long flight? Then maybe get more fuel from another planet's atmosphere or moon for the return trip?

Pretty neat article, thanks. But it doesn't seem to match your thread title, or your question about dipping into the atmosphere to "store" O2. The technology in the article appears to rely on a continuous high-velocity, low-density atmospheric flux to operate at low thrust values, no?

Yes, but the thrust motor, AFAIK, collects the molecules into a chamber as a low density gas as part of the process. At this point, some could be pumped off into a pressure vessel.
The thrust values are low, but if the orbit was a high elliptical one, there would be time to rebuild velocity and re-position for each skimming dip into the outer edge of the atmosphere.

Staff: Mentor

Yes, but the thrust motor, AFAIK, collects the molecules into a chamber as a low density gas as part of the process. At this point, some could be pumped off into a pressure vessel.
The thrust values are low, but if the orbit was a high elliptical one, there would be time to rebuild velocity and re-position for each skimming dip into the outer edge of the atmosphere.

I'm still confused. You used the word "fuel" in your title and the article is about reaction mass. Which one do you actually mean? By posting the article you imply your idea is similar, but it looks to me like it isn't.

The original article relies on solar power for the actual energy. If you dip into the atmosphere with solar panels of any significant size, they will fall off, or burn off.

You pretty much end up with a low-power solar-powered space plane (aerodynamically shaped solar "wings") with an ion engine that is only usable around Earth and Venus (no atmosphere on Mercury, not enough sunlight further out unless your wings are enormous) - or you end up with a flying nuke. And not an RTG kind of nuke, but a real nuclear reactor. Because, with current tech:
- a 24 kW (24 thousand Watt) HiPEP produces 460 mN of thrust (half a Newton), ie 1/20th of earth acceleration for a kilogram of payload.
- An RTG (radio-thermal generator) produces maybe 5 Watt of electricity per kilogram.
- So you need a 4800 kg machine for 1/20th kg of thrust.
(Yes, GOCE used solar, but it had 20 mN of Thrust.)

As for the nuclear option, I guess I will refer you to this thread: http://www.eng-tips.com/viewthread.cfm?qid=104814 . They end up with a 500kg reactor for 1MW power. That enables very high-power electric propulsion that reaches chemical propulsion forces. But you end up with an actual nuclear fission reactor on your ship.

I'm still confused. You used the word "fuel" in your title and the article is about reaction mass. Which one do you actually mean? By posting the article you imply your idea is similar, but it looks to me like it isn't.

If you dip into the atmosphere with solar panels of any significant size, they will fall off, or burn off.

Ah, yes, but if you look at the article, they have dealt with that already. The atmosphere is extremely thin, and the panels are on the sides of the craft. It's essentially orbiting in space, with just a little drag from a very tenuous gas.

only usable around Earth and Venus (no atmosphere on Mercury, not enough sunlight further out unless your wings are enormous) - or you end up with a flying nuke.

I wasn't thinking about nuclear. The craft in the article uses solar.
Solar can still work further out - the panels need to be bigger. They could collect power during the high phase of the orbit, it gets stored, then fold up the panels, dip into the atmosphere, run the pump on a little battery power, and go round again. And I think Mercury does have a very thin atmosphere. Maybe it's the right density for this to work?

Well, this idea is actually a basic Bussard Ramjet without fusion (and due the less effective 'gather' part it would rely on an atmosphere).

As long as you can give it enough power to counter the drag, it'll fly. But where would the power come from?

From photovoltaics, like in the article in the link.
As I understand it the Bussard Ramjet has not yet been built. But this thruster has, although it's not yet flown. I'm just thinking about a next-step innovation.

Perhaps, without the need to carry reaction mass for the return trip, a Mars return mission could be faster? Using Hall thrusters, that is.